The present invention relates to fabrication of heat pipes, and more particularly to a method of checking whether noncondensable gases remain in the heat pipe. The invention relates also to a process for producing heat pipes without allowing noncondensable gases to remain therein.
The heat pipe comprises a container and a condensable working liquid, such as water or PFC, enclosed in the container. If the heat pipe contains O.sub.2, CO.sub.2 and like noncondensable gases remaining therein, the working liquid fails to evaporate smoothly, impairing the performance of the heat pipe. Accordingly, it is required to fabricate heat pipes without permitting the noncondensable gases to remain therein to the greatest possible extent.
JP-B No. 78873/1994 discloses a process for producing such heat pipes. This process comprises providing an injection-closing nozzle at one end of a container in the form of a closed tube, evacuating the container through the nozzle, injecting a working liquid into the container through the nozzle, temporarily closing the nozzle at the outer end thereof, subsequently heating the container to evaporate the working liquid and thereby cause the nozzle to retain therein noncondensable gases within the container, thereafter completely closing the nozzle at its base end and cutting off a nozzle portion outward of the base end.
With this process, the noncondensable gases in the container are driven into the nozzle by heating and retained therein, whereas noncondensable gases are likely to remain in the heat pipe obtained since it is impossible to check whether the noncondensable gases in the container are completely retained in the nozzle.
Further because a major portion of the nozzle is removed, the process requires correspondingly increased material and working costs.
The process described is applicable also to the fabrication of a flat platelike heat pipe which comprises a container made from a clad metal plate having a tubular bulged portion, and a working fluid enclosed in the bulged portion. The container bulged portion is then provided with a working liquid inlet which is opened at an edge of the clad metal plate and which has connected thereto, for example, a metal tube serving as the injection-closing nozzle. However, the application of the process involves a problem, for example, when an increased amount of working liquid is injected into the tubular bulged portion. The working liquid will enter the nozzle from the interior of the bulged portion when bumped by heating the container, and a large amount of working liquid will be lost when the nozzle is subsequently cut off at its base end. To prevent the working liquid from flowing out in this way when the container is heated, it appears useful to provide a constriction in the tubular bulged portion in the vicinity of the liquid outlet, whereas difficulty will then be encountered in injecting the working liquid from the inlet.
JP-A No. 170889/1997 also discloses a process for producing a heat pipe so as not to allow noncondensable gases to remain in its interior. This process comprises injecting a working liquid into a closed tubular container having an injection tube at one end thereof, then temporarily closing the injection tube at a portion thereof toward its outer end, subsequently heating the container to evaporate the working liquid and thereby cause noncondensable gases within the container to be retained in the injection tube, detecting a boundary between the noncondensable gas portion and the working liquid based on a surface temperature difference of the injection tube in the lengthwise direction thereof, completely closing the injection tube in the vicinity of the boundary, and thereafter cutting the injection tube between the completely closed portion and the temporarily closed portion.
However, the position of the boundary is liable to shift according to production conditions, and the position at which the injection tube is completely closed is altered correspondingly. The injection tube portion remaining on the container after cutting then varies in length from pipe to pipe, consequently resulting in variations in the external size of heat pipes and possibly presenting difficulty in installing the heat pipes.
Because a major portion of the injection tube is removed, the disclosed process also requires correspondingly increased material cost and working cost.
Although the process is applicable also to the fabrication of flat platelike heat pipes, the same problem as is involved in the application of the process of JP-B No. 78873/1994 to the fabrication of such heat pipes will be encountered in this case.